Equatorial bubbles observed at the north and south anomaly crests: Dependence on season, local time, and dip latitude

Equatorial bubbles, which cause scintillation that disrupts transionospheric RF propagation, are difficult to study because they are unpredictable and extremely variable. This work undertakes to study them by continuous observation throughout a year at solar maximum using ionospheric sounders operating at sites in the western hemisphere in both north and south Appleton anomaly crests. Here direct observation in an extensive case study has found that bubbles can be identified when passing over a sounder near the Appleton anomaly crest by the appearance of range spread F (RSF) that is so severe that it prevents evaluation of f(o)F(2). Using this obscuring spread F as a signature, bubbles are recorded in the hourly tabulations of f(o)F(2) on 37 days a total of 104 times by three anomaly sounders at dip latitudes of 16 degrees and 20.3 degrees in the north and 19.9 degrees in the south. At each location, occurrence is maximum near the equinoxes, zero in July with a minimum in December, and maximum between 2200 and 2400 LT. Interpretation of the signature is supported by its consistency in both season and local time with bubbles observed by others. Overall, occurrence decreases by a factor of 0.38 between 16 degrees and 20 degrees dip latitude, apparently symmetrically about the dip equator. Whereas RSF at the anomaly always results from bubbles, RSF at the equator does not, the most evident period being near December when bubbles have a minimum but equatorial RSF has a maximum, occurring nearly every day following sunset. This nonbubble equatorial RSF apparently results from bottomside spread F.